1. Field of the Invention
The present invention generally relates to a four wheel drive system for an automotive vehicle, which either front wheels or rear wheels of the vehicle as main drive wheels while using remaining wheels as subsidiary drive wheels and which distributes a drive force supplied from an engine of the vehicle between the front and rear wheels. More specifically, the present invention relates to the four wheel drive system which is capable of changing of the drive force distribution for the front and rear wheels corresponding to the main and subsidiary drive wheels, in accordance with a drive mode selected manually by operation of a mode select lever or a mode select switch, or selected automatically based on a difference of rotating speed between the front and rear wheels.
2. Description of the Related Art
A part-time four wheel drive vehicle and a full-time four wheel drive vehicle are known as an automotive vehicle with such a four wheel drive system. The part-time four wheel drive vehicle enables a driver of the vehicle to select a drive mode between a two wheel drive mode and a directly connecting four wheel drive mode, by operation of a mode select lever or a mode select switch when either of the modes is required. On the other hand, the full-time four wheel drive vehicle automatically controls changing of the drive force distribution for the front and rear wheels corresponding to the main and subsidiary drive wheels.
In the part-time four wheel drive vehicle, when the two wheel drive mode is selected, the four wheel drive system makes the drive force distribution for the subsidiary drive wheels as 0% while making the drive force distribution for the main drive wheels as 100%, providing a two wheel drive condition. Also, then the directly connecting four wheel drive mode is selected due to, e.g. a rotating speed of the main drive wheels being faster than that of the subsidiary drive wheels, the four wheel drive system makes the drive force distribution for the subsidiary drive wheels as 50% while making the drive force distribution for main drive wheels as 50%, providing a directly connecting four wheel drive condition in which the the front and rear wheels are directly connected, so as to advance a fuel consumption ratio of the engine.
On the other hand, in the full-time four wheel drive vehicle, the four wheel drive system automatically changes the drive force distribution for the front and rear wheels corresponding to the main and subsidiary drive wheels in accordance with a difference of rotating speed between the front and rear wheels, so that the drive condition is continuously changed between the two wheel drive condition and the directly connecting four wheel drive condition.
Further known is a three mode four wheel drive vehicle which enables a driver of the vehicle to select an automatic four wheel drive mode except for the two wheel drive mode and the directly connecting four wheel drive mode, by operation of a mode select lever or a mode select lever switch. In the automatic four wheel drive mode, changing of the drive force distribution for the front and rear wheels is automatically controlled by the same way as the full-time four wheel drive vehicle.
As the four wheel drive system mounted on such a four wheel drive vehicle and capable of phased changing or continuous changing of the drive force distribution for the front and rear wheels corresponding to the main and subsidiary drive wheels, several kinds of systems are known. The known four wheel drive systems comprise, as a drive force distribution adjusting means, e.g., a variable torque clutch mechanism which is capable of controlling a variation of transmitting torque by controlling changing of fastening force, or a so-called limited-slip center differential gear mechanism which is capable of controlling a variation of transmitting torque by controlling a slip between front and rear drive shafts. The four wheel drive systems control the drive force distribution for the front and rear wheels by controlling the variation of transmitting torque by operations of such mechanisms.
As the variable torque clutch mechanism of the known four wheel drive system, a hydraulic control type clutch mechanism and an electro-magnetic control type clutch mechanism are mainly employed. The hydraulic control type variable torque clutch mechanism controls a fastening force by controlling changing of a friction force between clutch plates by controlling a hydraulic pressure fed to a clutch piston, so as to control the variation of transmitting torque. On the other hand, the electro-magnetic control type variable torque clutch mechanism controls a fastening force by controlling changing of a friction force between clutch plates by controlling an exiting current for a proportional electro-magnetic solenoid, so as to control the variation of transmitting torque.
In the aforementioned four wheel drive vehicles, a condition in which the main and subsidiary drive wheels can rotate at a different rotating speed may be changed to another condition in which the main and subsidiary drive wheels are directly connected and forced to rotate at a mutually same rotating speed, in accordance with selection of the directly connecting four wheel drive mode by a driver, or in accordance with an automatic changing to the directly connecting four wheel drive condition. For example, in the part-time four wheel drive vehicle, selection of the directly connecting four wheel drive mode for an advanced road-coverability results in changing of the drive condition from the two wheel drive condition to the directly connecting four wheel drive condition. Also, in the full-time four wheel drive vehicle, when escaping from a mire, the drive condition is changed to the directly connecting four wheel drive condition by e.g., the limited-slip center differential gear mechanism.
However, in a situation that a diameter of at least one wheel is smaller than diameters of other wheels of the vehicle due to e.g. an emergency wheel being used, it is required to avoid the directly connecting four wheel drive condition so as to enable the smaller diameter wheel to rotate faster than the other wheels. Because, when the vehicle becomes in the directly connecting four wheel drive condition in which the main and subsidiary drive wheels are directly connected and rotate at a mutually same speed under such a different diameter wheel using situation, a braking force acts relatively on the smaller diameter wheel to be rotated faster, due to a difference of circumferential speed of rotating wheel between the smaller diameter wheel and the other wheels, so that a cornering force of the smaller diameter wheel is varied in view of a friction circle, causing instability of running condition of the vehicle. Moreover, when the vehicle keeps running in the directly connecting four wheel drive condition under such a different diameter wheel using situation, a rotating speed difference between wheels following a backing movement of road surface and wheels driven by drive shafts is transmitted to the four wheel drive system, so that the four wheel drive power train system suffers an undesirable load.
Further, on e.g., a snowy road, during running of the aforementioned four wheel drive vehicles in a drive condition except for the directly connecting four wheel drive condition, when the vehicle is stuck in mud, and the main drive wheels idly rotate while the subsidiary drive wheels stop rotating, the full-time four wheel drive vehicle and the three mode four wheel drive vehicle in the automatic four wheel drive mode automatically change the drive condition to the directly connecting four wheel drive condition, because of the large speed difference between the front and rear wheels. Thereby, in some cases, the vehicle can escape from the mud. On the other hand, in such a situation, the part-time four wheel drive vehicle in the two wheel drive mode may change the drive condition to the directly connecting four wheel drive condition, result from selection of the directly connecting four wheel drive mode due to manual operation of, e.g., a mode select switch by a driver, so as to escape from mud.
However, when the drive condition of the vehicle is changed from the condition in which the main drive wheels idly rotating while the subsidiary drive wheels stopping rotating, to the directly connecting four wheel drive condition so as to escape from mud, the subsidiary drive wheels which stopped rotating become rotating as fast as the main drive wheels which idly rotating in high speed, so that, in some cases, a driver and/or a passenger feel uncomfortable, due to a shock caused by the rapid increase of a rotating speed of the subsidiary drive wheels.
In connection with the aforementioned four wheel drive vehicles, in case of the full-time four wheel drive vehicle and in case of the automatic four wheel drive mode being selected, the drive force distribution for the main and subsidiary drive wheels is determined in accordance with, e.g., a rotating speed difference between the front and rear wheels on the other hand, in case of the directly connecting four wheel drive mode being selected, the drive force distribution for the main and subsidiary drive wheels is determined as e.g. 50%:50%, Further, in case of the two wheel drive mode being selected, the drive force distribution for the main and subsidiary drive wheels is determined as 100%:0%, then a control signal in accordance with the determined value is sent to the drive force distribution adjusting means of the corresponding four wheel drive system, so that a drive force supplied from the engine is distributed for the main and subsidiary drive wheels, in accordance with the control signal.